Duplication Mutations

These mutations are an
important way that a genome can get
larger, and hence contain more information.

Let us imagine a sexually
reproducing species. Some gene G contains the information about
building some protein P. Some individual accidentally acquires a second
copy of G. If that isn't harmful, the mutation could spread through
the species, and by neutral
drift it could become the norm, or at any rate common.

At some later date, another individual has a mutation to one of his
two G's. The altered gene now causes protein P2 instead of protein
P. Suppose that P2 is different from P in a useful way. The individual
still manufactures P with his (or her) undamaged copy of G, but now P2
is manufactured too. The individual now contains new genetic information. Since P2 gives an
advantage, the new information has a very good chance of spreading
through the species. The species eventually comes to rely on P2's
special properties.

Essentially I am saying that a gene can get copied. And then, the
two copies are free to diverge - to mutate and become different -
without the creature promptly dying from loss of the old
function. Occasionally, the copy that diverges becomes beneficial.

The new protein P2 doesn't have to be wildly different to be
useful. For example, hemoglobin is the molecule which carries oxygen
in your blood, but a human fetus uses gamma-globin instead. The two
molecules are very similar, but the adult and fetal environments make
slightly different demands.

Could this scenario happen? Yes, because duplication
mutations are observed today:

"Actually, the process of gene duplication can occur in a number
of ways, and the most common mechanisms are well understood. Sexual
organisms, for example, have two sets of chromosomes (one from each
parent) which line up during the cell division process called
meiosis. As it happens, the very long DNA threads are constantly
breaking and being rejoined. The rejoining process is not 100 percent
accurate, however, and often one of the chromosomes comes away with a
little more of the DNA than its pairee, which will have
correspondingly less. The lucky gametes that come away with the more
are said to have had a "gene duplication," although the amount of DNA
may amount to only a part of a gene or maybe a whole string of
genes."

Do duplications harm the individual? Well, they can. Some cancers
seem to be caused this way, and the Charcot-Marie-Tooth neurological
disease seems to be caused this way. Thalassemias are caused by
having the wrong number of hemoglobin genes. (Some humans have more
than normal, and others fewer.) Duchenne dystrophies and cystic
fibrosis come from deletions, the opposite side of the coin from
duplication.

But some duplications don't seem to matter much, and some are known to
positively benefit the possessor. So
the scenario isn't asking for anything unknown or particularly
unlikely.

Is there evidence that this scenario has happened? Yes, it seems to
have happened often. Myoglobin, which stores oxygen in muscles,
strongly resembles hemoglobin, which carries oxygen in blood. Both are
necessary to humans, but invertebrates such as the worm
C. elegans only have one kind of globin. So, it is a reasonable
hypothesis that our genes for myoglobin and hemoglobin are "descended"
from one single ancestral gene, which got duplicated. And, in fact,
there are "unnecessary" structural similarities between human
myoglobin and human hemoglobin. If the similarities aren't due to
common ancestry, then we have found a really really big coincidence.

Lactalbumin is a component of milk. It and part of the lactose
synthetase enzyme are encoded by genes that differ only slightly
from the gene for lysozyme, which retards infection by
breaking down bacterial cell walls. So, a duplication and a little
tinkering can produce useful new objects from odds and ends.